Ammonium octamolybdate (AOM) is known primarily as a fire-retardant. The ability to use AOM to form an image on a substrate is disclosed in WO2002/074548.
Ammonium octamolybdate (AOM) is a pigment that undergoes a white to black colour change reaction on exposure to laser irradiation. It has become widely used in substrate coding and marking applications using lasers.
Freshly made AOM pigment powder typically comprises a large proportion of relatively small particles, known as ‘fines’. The presence of these fines is reflected in the DX values of the AOM pigment powder, where X is the percentage of particles below the quoted size, usually in microns.
With AOM pigment powder the consequence of the presence of a relatively large number of fines is it can adversely affect the rheological properties of any formulation subsequently made using the AOM pigment powder, such as liquid coatings, printing inks or plastic masterbatches.
In order to apply AOM pigment to a substrate for laser based coding and marking, such as those used in the manufacture of primary or secondary packaging, it must first be formulated into a liquid coating formulation or printing ink. The ink is then applied to the substrate using a standard industrial coating technique.
It has been observed that AOM pigment, when tested shortly after its manufacture, can give rise to liquid coatings or printing inks with rheological properties far from ideal. Usually the coating rheology is such that the liquid formulation is too thick. With liquid coatings/printing inks there is an ideal rheology window for application to a substrate. Optimum laser imaging performance requires the liquid coating/printing ink to have a specific minimum concentration of AOM pigment powder. Too little pigment causes laser imaging performance to be diminished. Too much pigment can adversely affect laser imaging performance by rendering the liquid coating/printing inks with poor rheological properties which reduces ink transfer during printing. Also having to use more pigment than is usually necessary makes the process less economically attractive.
For a given concentration of AOM pigment powder if the coating/ink is too thick the coating/ink cannot readily flow or transfer from the printing press to the substrate. This can be remedied by dilution with solvent. However, this causes the active pigment concentration to be diluted resulting in less pigment actually being transferred to the substrate resulting in diminished laser imaging performance. This is critical in applications requiring high contrast such as barcode readability. If the ink is too thin not enough will transfer and laser imaging performance will be diminished.
We have observed that the particle size of AOM pigment power can naturally increase with time. However, this process can take up to and over 12 months to fully complete. It is costly and impractical to hold large stocks of AOM pigment for such a relatively long time in an industry based on fast turnaround products.